This invention relates to a method of controlling an air-fuel ratio, and more particularly to a method of controlling an air-fuel ratio, suitable for use in an internal combustion engine for a vehicle, having an electronically controlled fuel injection device.
In an electronically controlled fuel injection device, a basic fuel injection time duration TP is computed on the basis of an engine speed NE detected by a rotational speed sensor and an intake air flowrate Q detected by an intake air flow sensor, and various correction are applied to the basic fuel injection time duration TP in accordance with the engine operating conditions so as to compute a final fuel injection time duration .tau.. A fuel injection valve is opened to inject the fuel for the final fuel injection time duration .tau..
On the other hand, in the fuel injection control device of the type described, in which CO, HC and NO.sub.x are to be simultaneously removed for the exhaust gas emission control measure, it is desired to control the air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio from the viewpoint of the effective removal of the above-mentioned three contents. Therefore, an oxygen sensor is provided in the exhaust gas path, and, under predetermined condition, the feedback correction coefficient FAF is computed so that the air-fuel ratio can approach the vicinity of the stoichiometric air-fuel ratio in accordance with an air-fuel ratio signal from the oxygen sensor, whereby the air-fuel ratio is feedback-controlled.
In the electronically controlled fuel injection device wherein the above-described feedback control of the air-fuel ratio, the air-fuel ratios under the predetermined conditions during the above-described feedback control are learned to compute learning correction coefficient FG in order to compensate a difference in the air-fuel ratio due to the variability of parts, compensate the air-fuel ratio for the running of the vehicle in the highlands (for the high altitude) and compensate a variation in the air-fuel ratio due to change of the intake air flow sensor with time.
For example, the final fuel injection time duration .tau. is obtainable through the following equation. EQU .tau.=TP.times.FAF.times.FG.times.K
where K is a correction coefficient determined by water temperature, intake air temperature and the like.
In learning the aforesaid air-fuel ratio, it must be taken in consideration that the fuel, which has evaporated from a fuel tank and has been accumulated in a canister (hereinafter referred to as the "evaporated fuel"), is fed to a combustion chamber under predetermined condition including that at least the throttle valve is not fully closed, and thus the air-fuel ratio becomes rich temporarily. The influence by the evaporated fuel upon the air-fuel ratio is as shown in FIG. 1. In an extreme case, the intake air flowrate Q becomes about 10% rich even in a region of a high air flowrate as high as 100 m.sup.3 /h. In consequence, if the operation of the vehicle is stopped immediately after the change in the air-fuel ratio due to the evaporated fuel as described above is learned, then the air-fuel ratio would become excessively lean when the vehicle is started again, thus presenting the disadvantage of lowered startability. For this reason, there is no need to learn the air-fuel ratio, which has become rich due to the evaporated fuel.
The compensation of the air-fuel ratio for the aforesaid high altitude prevents the air-fuel ratio from becoming richer. More specifically, since the higher the altitude is, the lower the air density becomes, the air-fuel ratio becomes richer when the vehicle runs at the high-lands. Therefore, in the compensation for the high altitude, the fuel injection rate is adapted to get less as the altitude becomes higher. The influence by the altitude of the high-lands upon the air-fuel ratio is substantially constant irrespective of the intake air flowrate as shown in FIG. 2. Because of this, in a region other than the region where the throttle valve is fully closed, it is difficult to attribute the air-fuel ratio being rich to whether the evaporated fuel or the altitude of the highlands.
On the other hand, when the intake air flow sensor is obstructed due to the change with time or the like, the deeper influence is exerted on the air-fuel ratio in the intake air flow rate being lesser, as shown in FIG. 3. There has been proposed the control method in which, when the air-fuel ratios between a region where the throttle valve is fully closed and a region other than the above differ by 15% or more, the obstruction of the intake air flow sensor is judged to be present and a subtraction is made from the learning correction coefficient so that the air-fuel ratio can be .lambda.(surplus rate of air)=1. In the conventional control method, the similar learning is carried out even when the difference of 15% or more of air-fuel ratio occurs due to the evaporated fuel affecting the air-fuel ratio in accordance with air flow rate as in FIG. 1. As a result, the compensation of the air-fuel ratio due to the change with time and the compensation of the air-fuel ratio due to the evaporated fuel are overlapped, so that it is difficult to carry out the compensation of the air-fuel ratio properly. Further, when the vehicle comes down from the highland with the throttle valve being fully closed, there is a possibility of that the obstruction compensation cannot be correctly carried out.